VFD Pump Relationships

7/31/2019 VFD Pump Relationships

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Guidelines for the Use of Variable Frequency Drive (VFDs)/Single Phase Generator

Combinations For Short-Term Operation of Small Water and Wastewater Pumping

Installations

Introduction

The use of Variable Frequency Drives for converting single phase to three phase powerhas generated a great deal of interest. This paper provides guidelines for using variablespeed controllers for operating small motors and pumps that require less than 15Horsepower.

In a VFD/Single Phase Generator application, the VFD is used to convert single phase tothree phase power fed by a single phase generator and to efficiently match a motor/pumpcombination at a lift station or a well to the conditions at the site. The use of a VFDallows water or wastewater service to be restored to minimum acceptable levels in a mosttimely, efficient and economical manner.

This paper will examine the centrifugal pump, electric motor, VFD and single phasegenerator relationships that are important in optimal selection equipment for this purpose.

Centrifugal Pump Relationships

Centrifugal pumps are widely used in water and wastewater treatment and exhibitcharacteristics that make them very adaptable to variable frequency control.

Generally centrifugal pumps are driven by electric motors that operate at synchronousspeeds of 1200, 1800 and 3600 RPM. The synchronous speed of a motor is based on arotation that is in perfect balance for a given voltage at a frequency of 60 Hertz. When amotor is placed under load, such as when it is connected to a pump, there will be someslip in the motor. Standard motor nameplate ratings are stated as 1140, 1725 and 3450RPM.

When the motor is connected to a centrifugal pump, the speed or the rotation inrevolutions per minute (RPM) where the pump operates, will provide a specific flow rateat a specific head condition. This unique condition, is called the centrifugal pumpsspecific speed. It may be calculated as shown below. From this relationship, a pumpcurve can be developed which will indicate how the pump will operate for various flowand head conditions.

Specific Speed (Ns) for a

Centrifugal Pump

Ns Specific Speed

rpm Speed in rpm

gpm Gallons per Min.

H Head in feet

Ns = rpm X (gpm)1/2

H3/4

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When the flow (gpm) and head (feet) of water are known. The theoretical horsepower foroperating the pump can be easily determined using the following formula. Since thespecific gravity of water is equal to one (1), it is often neglected.

Brake horsepower requirements gpm Flow in gpm

s gr. Specific Gravity for water

H Total Dynamic Head in feet

Bhp Horsepower required to drivepump

Bhp = gpm X H(ft) X s gr.

3960 X Pump Efficiency

Pumping equipment in water and wastewater operation will be operated at a constantspeed, or that speed provided by the motor. The variable frequency motor controller, is adevice thatchanges the frequency of the voltage applied to the motor, which in turnchanges the speed of the motor. When the speed of the motor changes, the applied

horsepower and the pumping characteristics also change. With a centrifugal pump, theserelationships are governed the Affinity Laws. A brief review of these relationships isprovided below.

Affinity Laws for a Centrifugal Pump Application

The operating conditions for a centrifugal pump may be estimated by using the AffinityLaws. The Affinity Laws are a group of relationships that may be used for estimatingFlow, Head Condition and Horsepower requirements of a centrifugal pump when thespeed of the pump is changed from a known speed or the specific speed, to some othervalue. The plot that follows governs the operating relationships for all centrifugal pumps.

Affinity Laws for Centrifugal Pumps:

Q1 = N1Q2 N2

Q Flow in gpm

N Speed in rpm

H Head in feet

Bhp Break Horsepower orThe Horsepower neededto drive the pump

H1 = ( N1 )2

H2 N2

Bhp1 = ( N1)3

Bhp2 N2

By using these relationships a plot for all centrifugal pumps for all pumping conditionscan be developed. These relationships will be used in controlling motor speed to providethe pumping characteristics needed in a variable speed control situation.

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Plot of Affinity Laws

for Any Centrifugal Pump Condition

100

80

70

60

50

40

30

20

10

0

% Q, Head, or Bhp

BrakeHP

Head

Capacity

0 10 20 30 40 50 60 70 80 90 100Rated Speed - %

The VFD will be used to change the motors operating characteristics from a constantspeed to a variable speed. Under different speed conditions, the pump will operate at adifferent capacity, different head or pressure output, and at a different horsepower. In thisapplication, the VFD operates the motor at a reduced horsepower.

Motor and Generator Sizing Relationships:

The Horsepower and Kilowatt requirements for supplying power to a motor can becalculated using the following formulas.

Motor HP and KW Requirements for a Pumping Condition:

HP = Bhp

Motor Efficiency X Power Factor

KW = .746 X HP

Note that the KW required for sizing a motor, is based on the brake horsepower needed tooperate the pump and calculated for a specific pumping condition. In the VFDapplication, the motor is already in place and sized to operate the pump. Since the VFDwill be used in an emergency situation where power must be supplied by a generator,what we need to know is the kilowatt rating of the generator needed to supply thishorsepower. Since there are efficiency losses by the motor and the power factor will be

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less than unity, a close approximation to the kilowatt rating of the generator needed tooperate the motor is the HP of the motor multiplied by 1000.

However, because starting current will be considerably higher than the full load currentrequirements it will be necessary to provide a larger generator than under these steady

state conditions. This is discussed in more detail in the following sections.

Adjustable Frequency Motor Control

Motors that drive pumps, must have sufficient horsepower to meet the requirementsimposed by the pumping unit. The horsepower that is required is the amount of force thatthe motor will have to move against. This force is described as torque. Torque is astandard measure for all electric motors and 1 ft-lb of torque is the turning force requiredto move a 1 lb. object at a distance of 1 foot away. Torque may also be described as thepounds of water that can be lifted by the pump/motor combination over a measureddistance and elevation.

Motors that are used in the water wastewater industry are classified as class B motors.They are designed to operate with standard, torque, horsepower, voltage and frequencyrequirements. It is these characteristics of class B motors that can be optimized by the useof a VFD.

As a motor starts it must overcome the inertia for the force at rest. This is known asstarting torque. As the motor is started there is a brief amount of time before the motorturns. At this instance a motor develops about 150% of its full-load torque. As the motoraccelerates the load, the torque will momentarily decrease then until it reaches amaximum of about 200% of its full-load torque. This is the accelerating or pull-up torque.If the motor is overloaded beyond its capability, the motor will stall or abruptly slowdown. This is referred to as breakdown torque. Torque will decrease rapidly as speedincreases until it reaches full-load torque. Full load torque is that torque produced whenthe motor is operating at its rated voltage, frequency and speed.

Current requirements for the motor for starting, pull-up, breakdown and full-load torquealso vary. When the motor is first started it momentarily consumes several times its fullload current. If a motor is started with no load applied to it, it will come up to its idlespeed. Even though no external force is applied, a motor under this condition willconsume about half its full load current. The importance here is that the motor under anyload condition will consume at least half its rated full load current.

Starting current, also known as locked-rotor current, is measured from the supply linewhen the motor is at rest just prior to acceleration. As the motor accelerates, the currentwill rapidly fall until the motor reaches full speed. The initial starting current or locked-rotor current can be 600% of the motors final full-load current requirements. A NEMA Bmotor that is started by connecting it to the power supply at full voltage (240V) and fullfrequency (60 Hz) will develop 150% of its starting torque and 600% starting current.

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The VFD operates the motor both at a reduced voltage and reduced frequency. The samemotor will now start at approximately 150% torque and 150% current. The VFD cansignificantly reduce the locked rotor current too because it allows the motor to operated ata reduced speed consuming only the horsepower necessary to move the load. The VFDallows the motor to gradually reach a set speed that is less than the speed of the constant

speed motor under normal operation.

Again, the VFD provides a considerable advantage over conventional equipment since itstarts a motor under a reduced torque and a reduced starting current. This VFD attributeallows a small generator to be used for small horsepower applications.

Variable Speed Motor Control Application

The full load current requirement is based on a motor delivering its rated horsepowerunder load. More current will be consumed if the motor is overloaded and less if underloaded. It is these relationships that are used in emergency variable frequency speed

(VFD) motor control applications.

The VFD is used to provide only the horsepower necessary to keep the load or maintainthe minimum pumping requirements, by reducing motor speed. In these applications, themotor will operate in this mode, until power is restored. The VFD is used to operate themotor efficiently under at a reduced load, at a reduced horsepower and thus at a reducedcurrent draw.

The formulas that govern motor horsepower requirements are shown below:

Formulas for Motor Horsepower Requirements:

T Torque

k Constantdepending onload conditions

F Frequency (Hz)

T = k X Volts X IWF

IW Current underload

N Speed in rpm

HP = T X N5250

.

With a VFD both the voltage and the frequency can be adjusted. As the motor is startedwith VFD it will gradually ramp to the speed needed to accelerate this load. TheHorsepower requirement can be exactly matched to the torque that the motor is requiredto move. In most all field applications encountered, the horsepower requirements neededby the pump will be less than the horsepower that has been supplied for the application.

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Single Phase to Three Phase Power Conversion

Another advantage of the VFD is that it can also convert single phase input voltage to

three phase output voltage required by a three phase motor. The VFD is most effectivein applications of small horsepower requirements that fall below 20 horsepower. Thiscondition applies to about 90% of the lift stations used in most municipal applications inFlorida.

Municipal applications almost always use three-phase power to motors for pumpingapplications. This is because of the high efficiency losses that occur as a result of the veryhigh amperage needed for single phase service and the larger equipment such as breakers,starters and wiring needed to accommodate it. For example, a 3 HP motor served withsingle phase service would require 34 amps compared to 11 amps for a three phase 3 HPmotor.

As previously discussed, when the pump is operating against lower head and/or reducedpumping conditions, the horsepower requirements are reduced. The lower horsepowerrequirements for the motor can then be directly matched using the VFD. This directlyaffects the amount of current that must be provided by the generator. For example, whenusing the 3 HP motor mentioned above that required 11 amps but actually using 2 HP, theamperage needed is reduced proportionally by the VFD with only about 8 amps required.

There are a few limitations in operating a motor at reduced frequency and voltage thatneeds to be mentioned. First is that all, according to NEMA Standards, Domestic motorscan be operated at 50 Hertz if voltage and horsepower ratings are appropriately reduced.Class B motors are designed to be operated at this reduced frequency. However, belowthis frequency heat will not be dissipated as fast since the motor is turning slower.Overheating of submersible pump motors have not been found to be a problem whenoperated as low as 40 hertz because of the higher heat transfer rate provided by movingwater.

However, operating at speeds below 30 Hertz causes other problems and may requireboost voltage to prevent motor damage. Thus, it is not recommended that a VFD ever beset to operate a motor below 30 Hertz.

Per manufactures recommendation, the VFDs should be de-rated by 25% or one motorsize. For example, it is recommended that the VFD rated for 10 HP be used for amaximum 7.5 HP installation and a VFD rated at 7.5 HP unit be used for a 5 HPinstallation, etc.

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Sizing Considerations for VFD Units

The plot below illustrates the torque requirements at full-load for a centrifugal pumpinstallation. It is prudent engineering to over design the motor/pump combination for theactual torque conditions as shown by the 100% point. The VFD allows the motor/pump

combination to operate at any point along the curve. Note that the useable part of thecurve is that portion of the curve at a speed greater than 50% speed of full speed. Thebasis of adjustable frequency (VFD) control is that changing the frequency of the inputvoltage changes the speed of the motor.

In emergency service VFD/generator installations, the pumping head requirements willbe lower than that encountered under normal conditions. This condition will be the directresult of damages that occur to residences and private businesses such as restaurants.These contribute no or lower flow or lower water demand. Additionally, under extremeemergency conditions, large numbers of people will have evacuated the area. The need tooperate small HP pumping units at full capacity is rarely encountered in these situations.

However, there are some instances such as a pump down of a wet well, filling of a watertank, etc. where full speed operation may be desireable.

Percent Full-Load Torque

Recommended VFD

Operating Range

Min Setting

Recommended Frequency Set-up for VFD Control

The actual HP requirements will be always be dictated by the actual torque requirements(head and pumping requirements) encountered in the field. The chart is intended to give agood starting point for VFD set up. Since the head conditions will typically be thecontrolling factor, a good starting point for a VFD has been found to be a speed settingaround 45 Hz for most installations.

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The table below provides a good estimate of the head and pumping conditions that can beaccommodated by changing the speed (N) resulting from the use of a VFD.

For example, at 42 Hz, a 5 HP motor/pump installation would theoretically operate at

about 1.8 HP, at 70% of its rated speed of 60 Hz, could pump against 50% of its ratedhead and provide 70% of its rated pumping capacity. At a speed of 48Hz, it would require2.5 HP and be capable of pumping against 65% of the design head conditions delivering aflow of 80% of the original design requirements.

Under field conditions the VFD can be set to a minimum value of 45 hertz, thengradually increased until the operating speed is raised enough to fully or partially open acheck valve. The speed is then increased to a desirable level that maintains a constantflow rate and level condition in the wet well.

Brake Horsepower Requirements for a Centrifugal Pump:Max. Head (% H) and Pumping Capacity (%Q)

For Speed Reduction (N) Changes (% Full Speed)

60 HzN=100%H=100%Q=100%Bhp

54 HzN=90%H=85%Q=90%Bhp

48 HzN=80%H=65%Q=80%Bhp

42 HzN=70%H= 50%Q=70%Bhp

36 HzN = 60%H= 35%Q=60%Bhp

1 .7 .5 .4 .2

2 1.3 1 .7 .5

3 2.0 1.5 1.1 .7

5 3.3 2.5 1.8 1.2

7.5 4.9 3.8 2.6 1.8

10 6.5 5.0 3.5 2.3

15 9.8 7.5 5.3 3.5

Recommendations for Sizing Single Phase Generators

The biggest advantage of VFD pump control is that small single phase generators that canbe obtained at any building supply company such as Lowes or Home Depot can be usedto power small water and wastewater facilities in an emergency situation. There are a fewconsiderations in selecting a generator since the kilowatt rating is not directly applicableto three phase motor application.

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It is always best to provide a generator sized larger than needed since the cost differenceis nominal but the additional power and start up capability may be needed. Fieldconditions may also require full horsepower operation. The suggested sizingrequirements are to multiply the HP of the installation by 1000 and then select a

generator in a range that provides at least a 50% safety factor. Below are some suggestedguidelines. Note that some of the recommendations show that the generator must supplyto a HP less than nominal to provide a 1.5 safety factor. In these cases the VFD would beset to a HP rating less than nominal nameplate rating on the motor. For example in thecase of a 7.5 HP motor, a 6.5 HP setting using a 10,000 watt generator would provide a1.5 safety factor. However, the power provided might not be enough to provide theneeded HP in the field at the lower setting. It would be much better to use a 12,500generator that would provide a 1.6 safety factor for the 7.5 HP and reduce the HP in thefield as appropriate. Although the smaller generator may work for a specific application,IT IS NOT RECOMMENDED THAT THE GENERATOR BE SIZEDINTENTIONALLY BELOW THESE RECOMMENDATIONS.

Single PhaseGeneratorSize (watts)

Approx.Cost

Approx.Weight

HPAppl.

SafetyFactor

5500 $1000 160 lbs. 3 1.8

6500 $1300 5 1.6+

10,000 $2000 300 lbs.


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Methods of Connecting the VFD in the Field

There are three recommended methods for connecting a VFD to a motor: 1.) direct to

power the pump panel, 2.) direct to the motor/pump using a separate control circuit, and3.) direct to the motor/pump with a constant setting. These are discussed below:

Direct Connection Using Existing Pump Controls

In this method of connection the line power is disconnected and the VFD is connected toone motor/pump input lead just below the breaker. The VFD is controlled through theexisting pump control panel using a relay that intercepts the motor starter command. Inthis mode 110V must be supplied by the generator to operate the motor controls. Makingthe connection in this way allows the lift station heaters to function.

Disconnections and re-connections to the VFD must be made at the motor starter and atthe motor three phase input connection. The secondary pump is shut off to limit thecurrent demand. The VFD can be used to operated the pump at reduced or full capacity asneeded.

Direct Connection Using Supplied Control Circuit

In this method of connection the VFD is connected to the motor input leads. A suppliedcontrol circuit consisting of two float balls is set up in the field. In this method ofconnection only the disconnection of the motor input leads and re-connection to the VFDis necessary.

Direct Connection Using No Control Circuit

In this method of connection the VFD is connected to the motor input leads. The VFD isset to control the motor at a reduced speed. Under this mode the motor will operatecontinuously.

In the direct connect to the motor/pump connection, the VFD is set at a low setting thatkeeps the wet well pumped down but supplies a continuous stream of water to thesubmersible pump/motor for cooling and lubrication. A VFD controller used in this modeis the simplest of the installation methods, and in an emergency mode following ahurricane, the VFD can be shut off at night to conserve fuel after pumping down thecollection system and restarted in the morning when use of the wastewater collectionsystem increases. This installation mode is the easiest and most common type ofinstallation of the VFD/generator combination.

This method can also be used in a rotation to service a number of lift stations or in aqueuing operations where the VFD/generator is rotated, servicing the most needed lift

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station and then moving to the next critical one. How the VFD/generator is used isdictated by the field conditions and how many units can be made available.

Connecting the VFD directly to power a pump panel will not allow the VFD to be usedbelow 60 Hz unless phase monitoring equipment is disengaged, and the VFD will not

provide a soft start. This is not the preferred connection method although it has been usedsuccessfully in limited applications generally where horsepower requirements are below3 HP.

Some Advantages Provided by the VFD/Generator Combination

In emergency operations, there will be multiple power outages to an overwhelmingnumber of lift stations. The conventional method used is to bring in portable generatorsand/or use pumper trucks to service smaller stations in a queuing method, i.e. servicingthe most critical and moving to the next most critical. Equipment and labor resources willbe quickly overwhelmed leading to stressful and potentially unsanitary and unsafe

conditions.

The use of the VFD/Single Phase Generator combination provides a simple andeconomical solution to these short-term problems.

Some of the advantages of a VFD/generator compared to using a conventional three-phase generator are illustrated in the table below.

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VFD/Single Phase Generator Comparison

to Conventional Three Phase Generator Use

For Lift Stations 20 HP and Smaller

NO. Small VFD/Single Phase Generator Conventional Three Phase Generator

1. Generator Cost and VFD cost less than$3,000

Generator for 30 KW Cost $13,000 to $15,000. Larger unitscan approach $30,000.

2. Generators in 5 HP range availablelocally.

Generator for 5 HP station special order large trailermounted item.

3. Several Small generators can betransported in the bed of a conventionalpick up truck.

Each generator must be pulled by a recommended tonvehicle.

4. Generator set up can be performed bymaintenance personnel.

Generator set up requires the services of a commercialelectrician

5. Generators run on gasoline that isgenerally in adequate supply locally.

Generators run on diesel fuel. Obtaining fuel and fueling inemergency situations can be difficult and require fuelingvehicles

6. VFDs are extremely versatile and canhandle ranges of smaller HP sizes.

Generators costs vary slightly in smaller sizes, thus largersized units are frequently used at small lift stations.

7. Small generators are portable and canbe handled easily by two employees

Large generators typically require crew of two to threepeople to set up and require special electrical skills

8. Small generators require minimalmaintenance and can be considereddepreciated in 3 to 5 years

Large generators require considerable specializedmaintenance, frequent exercise (recommended monthlyunder load) and require protected storage. Service life is 20years.

9. Generators can be obtained anddeployed easily by affected utility

Transport of large generators to emergency areas requiresoutside crews to housed in the areas for extended periods

10. Emergency response is immediateprotecting health and meeting needs ofcitizens for water and wastewaterservice

Emergency response lags as assessments of areas andmovement of equipment from long distances progresses.

11. Generators can be carried through backyards and placed in areas that areinaccessible to utility vehicles

Utility vehicles can not access some areas immediately afterstorms because of the amount of debris and trees that areknocked down.

12. Use of these generators frees largerunits for deployment elsewhere

Large generators supply is limited and priority applicationresults in some areas waiting extended periods for help.

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Frequently Asked Questions

1. What type of generators do I need and how many do I need for my system?

Lift stations that are fed from other lift stations must be provided with a methodof supplying auxiliary power or support such as a pump-around or tanker service.Lift stations that feed into a 12 or larger force main must be provided with anemergency generator by DEP regulations..

Lift stations larger than 3 horsepower will typically use 240V, three phase service.Small generators typically provide 120V, single phase power and can not be usedunless a phase converter (or VFD) is used.

The minimum size three phase generator generally available and provided inemergency operations are 40 KW and are usually much larger than necessary.

These units cost in the $20,000 to $25,000 range.

Although each system is different, experience indicates that at least one portablegenerator is needed to support a maximum of 7 stations if they are close togetherand can be continually serviced in a route. If the stations cannot be serviced in thismanner more generators will be needed.

2. How do I determine what size and how much will a single phase generator will

cost for a small lift station?

To determine the single phase generator needed first determine the largest motorto be operated in the station and multiply it by 1000. There is a table provided inthis paper that accommodates the required safety factor.

Single phase generators can be purchased for about $0.25 per watt with the pricedecreasing as the generator size increases. They can supply power to one pump inthe station connected to a VFD.

Single phase generators are locally available, economical, and can be easilymoved to a site using existing personnel and transport vehicles (van or pick up.)

3. How do I determine what size VFD I will need and what do they cost for a small

lift station.

VFD are rated by the range of minimum to maximum HP applied. Generally theranges are 0 to 5 HP, 5 to 10 HP and 5 to 20 HP. The VFD is downwardlycompatible, that is a 20 HP VFD will also operate down to a 4 HP motor. The costof the VFD runs about $650 for the smaller unit, $850 for the intermediate and$1450 for the larger unit (5 to 20 HP range.) VFDs act as a phase converter andconvert the 240V, single phase power to 240 V, three phase power.

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4. How do I determine the proper field setting for the VFD for a lift station

installation?

This paper provides some general guidelines for a starting point for system set up.

However, each set up is different and it usually takes a few minor adjustments in the fieldto provide an acceptable set point. The VFD comes equipped with an interactive menuscreen and keyboard that allows operator adjustment with very little instruction. Thereare only a few settings required and these are based on the size of the motor and use of itsnameplate data, if available. If the information is not available, standard electrical chartscan be used with the same results. The table below shows the typical settings used in aVFD/Single Phase generator application. There will be two wire connections and aground connection from the generator on the input side to the VFD, and three output anda ground connection on the output side of the VFD to the motor/pump being operated.

Typical Settings Used for a VFD

Single Phase Generator Installation

VFD Setting Description

Frequency Set for 60 Hz for USMotors

FLA Full Load Amp requirementfrom Nameplate or fromTable

Low Frequency Setting for OperatingFrequency for de-rating of

pumping capacityVoltage Input/Output Voltage ofVFD

Over Current Prot. Motor Protection, typicallyset at 1.5

Acceleration Allows motor to ramp tofull speed; setting 0 to 5seconds

Generally the VFD/generator will be used only in an emergency situation. In thissituation concern is providing a short-term means for a temporary water orwastewater electrical connection that will restore minimal service. TheVFD/generator installation is intended for that interim period of time when no linepower is available from the power company, until the time it is restored.

Attempting to provide full pump capability to every utility installation, in anemergency situation, is impractical because of the limited availability of three

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phase generators, the cost of transport, the cost of installation and servicing andthe labor requirements needed to manage an operation of this magnitude.

5. Where do I obtain a VFD and what Size VFD(s) and generator(s) do I need.

VFDs are manufactured by Square D and/or Telemecanique and are the Altivar 31model. These have been used successfully in testing in Naples Florida for thispurpose. They can be obtained from Grainger and other electrical supplycompanies. VFD units should be sized with a safety factor of 25% (downsize thehighest HP rating for the VFD by 25%, i.e. for a 7.5 HP application use a VFDrating of at least 10 HP.)

As discussed in the paper, FRWA recommends a generator sized by multiplyingthe HP at the location by 1,000 and adding a 50% safety factor.

6. Where can I obtain assistance with my installation?

Contact Florida Rural Water Association at 850-669-2748 or at frwa.net.

This paper was prepared by Robert McVay, P.E., Water Trainer for Florida Rural WaterAssociation. The paper presents experienced gained at installation in the City of Naples.Naples successfully piloted the use of single phase generators/VFD combinations in liftstation for emergency applications following hurricane Wilma that made landfall there.These units were also successfully used in Broward County during the 2005 hurricaneseason by Florida Rural Water Association.

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VFD Pump Relationships